Magnetic core



Patented Apr. 28, 1953 MAGNETIC CORE J acob. Louis Snoek, Jan Jacobus Went,and Pleter 'Willem Haayman, Eindhoven, Netherlands, as-

signors, by mesne assignments, to Hartford National Bank and Trust Company, Hartford,

Conn as trustee No. Drawing. Application April 10, 1946, Serial No. 661,076. In the Nethcrlands'J'ulyfi, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires July 6,1964

6; Claims.

This invention relates to magnetic cores containing; a ferro-magnetic ferrite as magnetic material. More particularly, theinvention relates to magnetic cores-for use at lowvalues of magnetic induction; for example in radio; telegraphy and telephony applications.

For various purposes it is of importance to have a material possessing a low value of its longitudinal m'a'geto-striction. As employed throughout the specification this is the absolute value of" the magnet'o -striction.

Thus; for example; a low magneto-striction is one requirement for obtaining low hysteresis losses, such as is desired e. g. for filter coils and Pupin coils. Furthermore; alow magneto-striction is important for attaining a high initial permeability;

The invention permitsobtaining very low values of the: magneto-striction with ferrites by providing: that the ferrite consists of a mixed crystal of a ferrite with positive magneto-striction and a, ferrite with. negative magneto-striction in a suitable mixing ratio.

For a betterunderstanding of the invention it-is pointed out, that, as has now been found, by far the greater part of the fer-rites have-negative values in regard to magneto-striction. A ferrite with positive magneto-striction is, for instance, magnetite (F6304 or FBOFGzOs). By combining 'ferrites with negative magneto-striction and a ferrite with positive magneto-striction it is possible to obtain mixed crystals of which the: values in regard, to magneto-striction. are between those of the mixed: crystal. compon n s. Byregulating' the: content. of. the component. with positive inagneto-strictimr it has, according to the invention, been found, possible to vary the magneto-striction-and to obtain a very low'va-lue thereofi, either positiveor negative, or: practicaly qualize; zero.

It is to: be: noted, that, as is Well-known, the magneto-stri-ctiona of. a metal acquires very low values at; a temperature. just below the: Curie a low valueand', inpracticeuas is well known, the

quotient.

at saturation and is defined as the magnetic moment per unit volume, or as the pole strength per unit area both being equivalent and having a unit value of maxwells per square centimeter; and A is the magneto-striction constant which isa ratio and is the change in length per unit length of a ferromagnetic body which is subject to an external field which will saturatev the magnetic material, is more important than the value of A itself. For instance, the initial permeability of a magnetic material is represented by the formula 2 max (Becker. and Doring, Ferromagnetismus, 1939, page where 0' represents the average value of the stresses inthe material and is defined as a force per unit area which in the c.g.s. system would have, a unit value of dynes per square centimeter.

Furthermore, the use" of materials whose Curie point just exceeds the ambient temperature of the material chiefly involves the drawback that the initialpermeability just below the Curie point largely depends on the temperature so that such material is unsuitable for practical applications. In fact, the variation of the initial permeability of a ferrite with temperature is usually such that with adecrease in temperature from the Curie point the permeability initially rises very quickly-to a. maximum and, upon a further decrease in temperature, gradually falls off. In practice, the range in which the permeability largely depends. on the temperature will be avoided and for this reason materials will always be used Whose. Curie temperature is at least to 50 C. Preferably, materials are used having a Curie point between 50 and 250 C. In the present case they expression Curie point is to be understood to mean the temperature at which. the initial permeability has dropped to a small fraction of about. 10% of the maximum value, or in other words, the temperature at which a magnetic material passes over into a state which can be conceived1 as having no ierromagnetism for all practical purposes, i. e. the magnetic properties ofthe: material havingjfall'en tosuch a value that the.- material is no, longer considered ferromagnetic.

By mixed" crystal formation of ferrites with positive and negativemagneto-striction according to the invention it is now possible to obtain a ferrite having a Curie-temperature of at least 50C., which consequently has. a sufficiently high 'value for-linax in practice and a low value of'the magneto-striction in such manner that the absolute value of the quotient is larger than 2x10 In the embodiment of the invention in which magnetite is used to constitute the mixed crystal component with positive A, the content of this component, instead of sintering together a ferrite with negative A and the ferrite with positive A in a suitable mixing ratio, or of sintering together a corresponding mixture of the component oxides, is preferably obtained by heating the ferrite with negative 7\ at a high temperature, for instance 1000 C. and upwards, in a suitable atmosphere, so that a certain quantity of bivalent iron is formed which corresponds to the desired quantity of magnetite; for if in a ferrite a part of the iron passes over, while splitting off oxygen, from the trivalent form into the bivalent, this can be conceived as a formation of Fesor in the ferrite. In this embodiment of the invention use is preferably made of a ferrite having a content of more than 50 mol. per cent iron, compute... on the basis of F6203 i. e. that the mixture, from which this ferrite has been formed by sintering, contained more than 50 mol. per cent of F6203. The excess iron xide may then serve for the conversion into magnetite; in the absence of an excess there would be a risk, in the formation of bivalent iron, of separation of one or more of the oxides other than F6203 constituting the ferrite, so that no homogeneous mixed crystal would ensue.

Theoretically, the atmosphere in which the ferrite is heated should have an oxygen contures exceeding about 1200 C., the composition of the atmosphere is relatively unimportant Whereas the time and the temperature of the heating are important, provided that the ferrite is not too strongly sintered to compactness so that splitting off of oxygen throughout the ferrite is possible. Care should be taken that during the cooling following the heating operation no interaction can occur between the ferrite and the gaseous atmosphere, due to which the outer layers of the ferrites would acquire a composition different from that of the inner part of the ferrite. Such inhomogeneites may be ascertained by removing the outer part of a ferrite core, for instance, by grinding, and by observation of the modification taking place in the properties of the core. To avoid the formation of a surface layer having a different composition the cooling may take place in an inert, oxygen-poor atmosphere 1. e. containing less than 2% of oxygen, for instance nitrogen,

or rapid cooling may take place after heating. As an alternative such a possible surface layer may be removed by grinding. i

It is pointed out that, as is well-known, several ferrites split off oxygen at a high temperature i. e. that such ferrites have a certain content of bivalent iron. It was not known, howthree hours in an iron ball mill.

ever, that the magneto-striction is affected by modification of the ferrous iron content. More particularly with these ferrites the ferrus iron content was not such that exceeded 2 10 throughout the ferrite.

From French patent specification 887,083 it is known to provide, with ferrites, for as high an oxygen content as possible i. e. as small a ferrous iron content of the ferrite as possible in order to reduce the losses of the magnetic material (eddy current, hysteresisand other losses). Although, according to the invention, in the manufacture of mixed crystals containing magnetite a certain ferrous iron content is provided for it has turned out that, in general.

the ferrous iron contents with which the magneto-strictions are low are not so large as to involve losses to a degree rendering the material impracticable particularly at low frequencies e. g. for telegraphy and telephony purposes (Pupin coils, filter coils).

The term manganese ferrite-forming constitutent is to be understood to include manganese ferrite, and the constituent oxides thereof.

The term nickel ferrite-forming constituent" is to be understood to include nickel ferrite and the constituent oxides thereof.

The term zinc ferrite-forming constituent is to be understood to include zinc ferrite and the constituent oxides thereof.

The term magnetite-forming constituent is to be understood to include magnetite and iron oxides capable of forming magnetite.

The ferrous ferrite materials have the formula:

in which the coemcients a, b, and 0 have values at which mBX exceeds 2 10 and the Curie point is greater than 50 C.

V Example 1 A mixture of pure MnOz, pure ZnO and pure F8203 in a mol. ratio of 25:25:50 is ground for From the mixture; with water as a plastification agent, a rod '100 x 2 x 2 mm. is formed at a pressure of 4 max the Curie point and the initialpermeability of these rods figure in the table below.

Gurie= point p in 0.

Iran in 0. g. S. units From the material corresponding with the preparation No. 2 a magnetic annular circuit having an effective permeability of 125 is constructed. At 2000 cycles/sec. and a maximum induction of 7.5 Gauss the value lit for this circuit amounted to 0.24, Rh being the hysteresis resistance of the coil wound on the annular core and L being its self-induction.

Example 2 Three mixtures of pure NiO, pure ZnO and pure F8203 in mol. ratios of 15:35:50, 1332322254 and 12.6:29.4:58 are formed into ferrite rods similarly to Example 1. The magnetic properties of the materials thus produced are given in the table below, in which the content Of bivalent iron is also given.

Ferro- Inn: in 1m 2 Curiecontent A.l c. g. s point in perunits A in 0. cent by weight lot 1, -2.5 300 3. 6 150 1.08 lot 2, -0.7 350 17.5 200 2.85 lot 3, +4.1 400 4 300 5. 38

What we claim is:

1. A soft ferromagnetic material consisting es sentially of substantially homogeneous mixed crystals of manganese-zinc-ferrous ferrite, said ferrite having the formula:

the coeflicients a, b and 0 being mol fractions and having values at which the Curie point is at least 50 C. and the value of the coefiicients a, b, and 0 being mol fractions and having the values at which the Curie point is at least 50 C. and the value of max is greater than 2X10, Imax being the magnetization at saturation in maxwells per cm. and A the magnetostriction value in change in length per unit length of the material.

3. A process of manufacturing a soft ferromagnetic material comprising the steps of mixing a manganese ferrite-forming constituent, a zine ferrite-forming constituent, and a magnetiteforming constituent in amounts expressed in terms of mol percents of M1102, ZnO, and FezOs of about 21 to 25, 25 and about 50 to 54, and producing upon sintering substantially homogeneous mixed crystals of manganese-zinc-ferrous ferrite having a bivalent iron content at which exceeds 2X10" Imax being the magnetization flux at saturation in maxwells per cm. and i being the magnetostriction value in change in length per unit length, sintering the mixed constituents at a temperature of about 1000 to 1300 in an inert oxygen-poor atmosphere for a sufficient time to produce said mixed crystals, and cooling the mixed crystals in said atmosphere.

4. A process of manufacturing a soft ferromagnetic material comprising the steps of mixing a nickel ferrite-forming constituent, a zinc ferriteforming constituent, and a magnetite-forming constituent in amounts expressed in terms of mol. percentages of N10, ZnO and F6203 of about 12.6 to 15, 29.4 to 35, and 50 to 58 and producing upon sintering substantially homogeneous mixed crystals of nickel-zinc-ferrous ferrite having a bivalent iron content at which max exceeds 2X10 Imax being the magnetization flux at saturation in maxwells per cm. and 7\ being the magnetostriction value in change in length per unit length, sintering the mixed constituents at a temperature of about l000 to 1300 C. in an inert oxygen-poor atmosphere for a sufficient time to produce the mixed crystals, and cooling the mixed crystals in said atmosphere.

5. A process of manufacturing a soft ferromagnetic material comprising the steps of mixing about 21 to 25 mol. per cent of MnOz, about 25 mol. per cent of ZnO, and about 50 to 54 mol. per cent of F8203, sintering the mixture at about 1300 C. in an inert oxygen-poor atmosphere for a suflicient time to form substantially homogeneous mixed crystals of manganese-zinc-ferrous ferrite having a bivalent iron content at which max exceeds 2x10 Imax being the magnetization flux at saturation in maxwells per cm. and A the magnetostriction value in change in length per unit length, and cooling the mixed crystals in said atmosphere.

6. A process of manufacturing a soft ferromagnetic material comprising the steps of mixing about 12.6 to about 15 mol. per cent of NiO, about 29.4 to about 35 mol. per cent of ZnO, and about 50 to about 58 mol. per cent of F6202, sintering the mixture at about 1300 C. in an inert oxygen-poor atmosphere for a sufiicient time to form substantially homogeneous, mixed crystals of nickel-zinc-ferrous ferrite having a bivalent iron content at which la-X exceeds 2X10 Imax being the magnetization flux at saturation in maxwells per cm. and 1 magnetostriction value in change in length per unit length, and cooling the mixed crystals in said atmosphere.

JACOB LOUIS SNOEK. JAN JACOBUS WENT. PIETER WILLEM HAAYMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,946,964 Cobb Feb. 13, 1934 2,452,529 Snoek Oct. 26, 1948 2.452.530 Snoek Oct. 26, 1948 

1. A SOFT FERROMAGNETIC MATERIAL CONSISTING ESSENTIALLY OF SUBSTANTIALLY HOMOGENEOUS MIXED CRYSTALS OF MANGANESE-ZINC-FERROUS FERRITE, SAID FERRITE HAVING THE FORMULA 